call sqlite3_column_blob() before sqlite3_column_bytes()

sqlite3 documentation states sqlite3_column_blob could modify the
the content and recommends the "safest and easiest" policy is to
invoke sqlite3_column_blob() followed by sqlite3_column_bytes()

from: http://www.sqlite.org/c3ref/column_blob.html
Merge pull request #67 from dajohi/master

call sqlite3_column_blob() before sqlite3_column_bytes()